If you don't periodically prune the database of deliveries, you can fill up your file system. Adding a cron job which will run once a day is one way to do this. That involves going to /etc/cron.daily and creating a file named cyrus-imapd. Inside that file, put the following two lines of code:

#!/bin/bash
su cyrus -s /bin/bash -c '/usr/cyrus/bin/deliver -E 3'

Make this script executable by running: chmod 755 cyrus-imapd

You should also create the deliverdb directory to store database files in: